Re: [PATCH 7/8] uprobes: perform lockless SRCU-protected uprobes_tree lookup

[Oleg Nesterov]

On 08/07, Andrii Nakryiko wrote:
>
> So, any ideas how we can end up with "corrupted" root on lockless
> lookup with rb_find_rcu()?

I certainly can't help ;) I know ABSOLUTELY NOTHING about rb or any
other tree.

But,

> This seems to be the very first lockless
> RB-tree lookup use case in the tree,

Well, latch_tree_find() is supposed to be rcu-safe afaics, and
__lt_erase() is just rb_erase(). So it is not the 1st use case.

See also the "Notes on lockless lookups" comment in lib/rbtree.c.

So it seems that rb_erase() is supposed to be rcu-safe. However
it uses __rb_change_child(), not __rb_change_child_rcu().

Not that I think this can explain the problem, and on x86
__smp_store_release() is just WRITE_ONCE, but looks confusing...

Oleg.